Here and hereinafter, ‘innerliner’ is intended to mean an inner layer of rubber substantially impermeable to air and used in tubeless tyres, i.e. tyres with no inner tube, to maintain air pressure inside the carcass.
All the portions listed above must be made as impermeable as possible to oxygen.
The innerliner must confine the oxygen as far possible inside the carcass, and prevent it from diffusing inside and degrading the compounds from which the other parts of the tyre are made.
As regards the tyre portions incorporating metal reinforcing cords, correct adhesion of the polymer base of these portions to the cords embedded in them is essential to ensure correct performance of the tyre. In fact, the working life of a tyre may be impaired by a reduction in adhesion of the polymer base to the cords.
Oxidation-induced degradation of these portions has been found to have a negative effect precisely on adhesion of the polymer base to the cords, and to eventually result in detachment of the polymer base.
The compounds from which the innerliner is made normally have a matrix of isobutyl rubber.
As is known, the tyre industry needs to make increasingly thin impermeable layers, known as innerliners, but without impairing performance in terms of impermeability. A thinner innerliner mainly means less material is used, which has obvious advantages in terms of production, and in reducing tyre weight, overall energy consumption of the vehicle, and rolling resistance.
One way of enhancing the impermeability of the innerliner without necessarily making it thicker is to use special fillers in the compound, which, when appropriately mixed, form a steric hindrance that greatly improves the impermeability of the innerliner as the end product. In other words, when mixed with the polymer base, fillers such as clay, kaolin, mica, etc. form an air barrier in the end product, preventing air from flowing through the product and so improving its impermeability. In this connection, it is important to note that any anisotropy of the filler may even further improve the impermeability of the rubber.
Another solution is to add polymer materials with a high glass transition (Tg) to the compound. A typical example is PET, widely used in the food and drink packaging industry.
Using conventional technology, however, mechanically bonding the fillers or high Tg polymers, such as PET, to rubber often poses serious problems.
Oxygen diffusion inside portions incorporating metal reinforcing cords may be prevented by increasing the thickness of the innerliner to form a better oxygen barrier. As stated, however, this solution has the drawback of invariably increasing rolling resistance and production cost.
Another solution is to use materials that improve adhesion of the compound to the cords by reducing the effect of oxidation-induced degradation. In the case of metal cords, for example, the presence of oxygen, in addition to degrading the rubber through oxidation, also oxidizes the metal, thus aggravating detachment of the polymer base from the cords. In these cases, cobalt salts are known to be used in the compounds as adhesion promoters, but these have the drawback of increasing tyre production cost.